Fuel cell powerplant employing an aqueous solution

ABSTRACT

A fuel cell powerplant 10 having a flow path 20 for an aqueous solution includes a device 18 for disposing in the solution ferric hydrous oxide of a character that retards the deposition of iron based compounds. The device is disposed within the powerplant and receives water from a conduit 42 which communicates with a component of the fuel cell powerplant, such as a fuel cell stack 12.

CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to U.S. application Ser. No. 745,464, filedJune 17, 1985 for APPARATUS EMPLOYING AN AQUEOUS SOLUTION by Albert P.Grasso et al.; U.S. application Ser. No. 745,465, filed June 17, 1985for FUEL CELL POWERPLANT EMPLOYING AN AQUEOUS SOLUTION by William A.Taylor; and U.S. application Ser. No. 746,168, filed June 18, 1985 forFUEL CELL POWERPLANT EMPLOYING AN AQUEOUS SOLUTION by Albert P. Grassoet al.

DESCRIPTION

1. Technical Field

This invention relates to fuel cell powerplants which use an aqueoussolution as a working medium. The working medium may be used to removeheat from components or to produce steam for processing raw fuel.Although this invention was developed for use in the field of fuel cellpowerplants, the invention may have application in any field employingaqueous solutions that deposit iron based compounds on the walls of theconduit.

2. Background of Invention

Fuel cell powerplants produce electric power by electrochemicallyconsuming a fuel and an oxidant in one or more electrochemical cells.The oxidant may be pure oxygen or a mixture of gases containing oxygen,such as air. The fuel may be hydrogen. One source of hydrogen is a fuelprocessor that reforms natural gas or any appropriate hydrocarbon byusing heat and steam to crack hydrocarbons.

Typically, a stack of fuel cells is used in performing theelectrochemical reaction. During the electrochemical reaction, the fuelcell stack produces electric power, a reactant product and waste heat. Acooling system removes waste heat from the stack. The cooling system mayadvantageously use an aqueous coolant to provide both waste heat andwater (as steam) to the fuel processor.

The cooling system includes flow paths for the aqueous coolant which arebounded by conduits. The conduits extend to the steam separator and tothe fuel cell stack for ducting the coolant to critical locations. Theseconduits may have small orifices for controlling the distribution ofcoolant throughout the cooling system.

One problem with aqueous coolants is the cumulative deposition ofparticles on the walls of conduits. The particles may occur as ions oras minute parts of matter. The particles, which are capable ofaccumulation to the point of blockage, are generally iron basedcompounds. These iron based compounds are composed mainly of iron basedoxides, such as magnetite and hematite, iron based salts, such as ironphosphates, and other compounds which result from the corrosion of ironincluding certain ferric hydrous oxide particles (hereinafter type Iferric hydrous oxide). The iron based compounds may form as the coolantcomes in contact with materials containing iron. Such contact mightoccur as the coolant is flowed through conduits in the powerplant orthrough supply conduits to the powerplant.

The problem is particularly troublesome for cooling systems using smallorifices because the particles may block the orifices. Any blockage ofan orifice in a fuel cell stack, for example, will increase the flowresistance through the stack and may even cause an inadequate supply ofcoolant to a critical location within the fuel cell stack.

One way of establishing the effect of such deposition on flow resistancethrough the stack is to treat the supply conduits of the fuel cell stackas if they were an equal number of equally sized ideal orifices. Thediameter of these equally sized orifices is called the equivalentdiameter of the fuel cell stack. The equivalent diameter may be foundexperimentally as follows:

1. Establish a constant flow rate of coolant through the fuel cellstack.

2. Measure the pressure drop through the fuel cell stack.

3. Calculate an equivalent diameter for the stack (or any system) by theequation

    De=2.8(W.sup.2 /ΔPN.sup.2 d).sup.1/4 10.sup.-7

where:

De=Equivalent Diameter, inches;

W=Total Coolant Flow (pound per hour);

d=Coolant Density (pounds per cubic foot);

N=Number of Conduits in Cell Stack;

ΔP=Differential Coolant Pressure Across Cell Stack (pounds per squareinch, difference).

FIG. 2 is an example of the effect such depositions can have on theequivalent diameter of a fuel cell stack under actual operativeconditions. After twenty-five hundred hours of operation (curve A) theactual equivalent diameter Dea was less than seventy percent of theinitial equivalent diameter Dei. The decrease in size and the resultingdecrease in the flow rate of coolant required a shutdown of the fuelcell stack for cleaning.

The fuel cell stack was cleaned after this period of operation byflowing a pressurized, acidic solution through the conduits. Cleaningrestored the actual equivalent diameter Dea to ninety-five percent ofthe initial equivalent diameter Dei (curve B). After another twenty-twohundred hours of operation (curve B), the actual equivalent diameterdecreased to less than seventy percent of the initial equivalentdiameter. Again, both the decrease in size and the reduced coolant flowrate required the shutdown of the fuel cell stack for a second cleaning.The fuel cell stack was also shutdown for other reasons at 3,700 hoursof operation (curve B) before the second cleaning. After restarting thepowerplant, the equivalent diameter recovered for a short period(approximately 250 hours) before decreasing again. It is theorized thatthe recovery is connected with transient conditions in temperature andflow rate which occur during a shutdown and start-up of the fuel cellstack. As shown, the effect is temporary.

The powerplant was cleaned again for a second time. After less thansixteen hundred hours of operation (curve C), the equivalent diameterdecreased to almost seventy percent of the initial equivalent diameter.

These periodic shutdowns and cleaning operations which result fromparticle deposition are both time consuming and costly.

Several approaches have been suggested for solving the problem ofparticle deposition from an aqueous coolant. One suggested approach isto reduce the amount of particles (including particles that are ions) byproviding a purified aqueous coolant, suppressing corrosion by raisingthe coolant's pH to high levels consistent with materials used inconstructing the system and by reducing the dissolved oxygen levelsbelow forty parts per billion (40 ppb).

Chemical additives are used in highly contaminated solutions to promotethe formation of sludge which is periodically removed from the system.

Another suggested approach for reducing the amount of particles is touse an aqueous coolant having a pH of about 6 to 8. Moderate levels ofdissolved oxygen are permitted in the water (40-400 ppb) to suppresscorrosion. Chemical additives are generally avoided.

Each of the above-mentioned methods utilizes a controlled flush ratefrom the system, called blowdown, which is necessary because corrosioncannot be totally eliminated and chemical cleaning is eventuallyrequired. The aqueous coolant that is lost is replaced by addingcoolant. The added coolant is commonly called feedwater.

Despite the existence of these techniques for controlling the amount ofiron based compounds in cooling systems having an aqueous coolant,scientists and engineers are seeking to develop additional ways ofdirectly blocking the deposition of iron based compounds on the walls ofconduits.

DISCLOSURE OF INVENTION

According to the present invention, an aqueous solution for use in aconduit of a fuel cell powerplant consists essentially of water thatmeets certain specifications and that contains an amount of ferrichydrous oxide of a character that retards the deposition of iron basedcompounds on the interior of the conduit (hereinafter type II ferrichydrous oxide).

A feature of the present invention is water having a pH which is atleast 5.5, an electrical conductivity which is less than or equal to onemicromho per centimeter and a solids content less than one part permillion that includes an amount of iron based compounds other than typeII ferric hydrous oxide. Another feature is ferric hydrous oxidedisposed in the water, the ferric hydrous oxide being of a characterthat retards the deposition of iron based compounds on the interior ofthe conduit (type II ferric hydrous oxide).

The foregoing features and advantages of the present invention willbecome more apparent in light of the following detailed description ofthe best mode for carrying out the invention and in the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of an actual fuel cell powerplanthaving a cooling system which used an aqueous solution as a coolant andas a source of steam and further shows an auxiliary system for supplyingtype II ferric hydrous oxide to the powerplant under operativeconditions.

FIG. 2 is a graphical representation of the equivalent diameter Dea ofthe fuel cell stack versus run time under operative conditions withoutthe addition of type II ferric hydrous oxide; the equivalent diameter isnormalized by dividing by the initial equivalent diameter Dei.

FIG. 3 is an enlarged, cross-sectional view of an orifice used in thecooling system of FIG. 1.

FIG. 4 is an alternate embodiment of the fuel cell powerplant shown inFIG. 1.

FIG. 5 is an enlarged schematic representation of a portion of thepowerplant shown in FIG. 4.

FIG. 6 is a graphical representation of the equivalent diameter Deaversus run time under operative conditions with the addition of type IIferric hydrous oxide; the equivalent diameter is normalized by dividingby the initial equivalent diameter Dei.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a schematic representation of a fuel cell powerplant 10 whichincludes a fuel cell stack 12, a fuel processing system 14, a coolingsystem 16 and a coolant recovery and supply system (CRS) 18.

The cooling system 16 has a flow path 20 for coolant which, in theembodiment shown, is an aqueous solution. A means for disposing type IIferric hydrous oxide in the aqueous coolant, such as conduit 22, is incommunication with the flow path 20. As shown, the conduit 22 is part ofan auxiliary system 24 for supplying type II ferric hydrous oxide to thepowerplant under laboratory conditions. Alternatively, the conduit 22might be in flow communication with the coolant recovery and supplysystem 18 at location 26.

The fuel cell stack 12 includes a plurality of electrochemical cells asrepresented by the two electrochemical cells 28. Each electrochemicalcell has an anode 30, a cathode 32, and an electrolyte 34 disposedbetween the anode and cathode. The electrolyte places the anode in ioniccommunication with the cathode. Air is supplied to each cathode viaconduit 36. Processed fuel is supplied to the anode via conduit 38. Aportion of the fuel is combined electrochemically with a portion of theoxygen in the air via the electrolyte to produce electric power. Theremaining processed fuel is exhausted from the anode and flowed viaconduit 40 to the fuel processing system 14 where the fuel is combusted.The remaining oxygen is exhausted from the cathode and merged with theanode exhaust via conduit 42.

The fuel processing system 14 includes a fuel processor 44. The fuelprocessor receives steam via conduit 46 and a raw fuel, such as naturalgas, via conduit 48. The fuel processor catalytically combines the steamand the raw fuel to produce a processed fuel such as hydrogen. Becausethis reaction is endothermic, the fuel processor has a burner (notshown) for generating heat. The unreacted process fuel from the stack isburned in this burner.

The cooling system has a coolant pump 50 for circulating pressurizedaqueous coolant, a heat exchanger 52 for removing heat from the aqueouscoolant and a steam separator 54 for separating steam from the aqueouscoolant.

Under normal operating conditions, the temperature of the aqueouscoolant generally reaches approximately three-hundred and fifty degreesFahrenheit (350° F.; 180° C.) and in powerplants may typically rangebetween 160° C. to 205° C. with an oxygen concentration ranging between20-400 ppb with the most likely range being between 40-80 ppb. Steamseparated from the aqueous coolant in the steam separator 54 is flowedvia conduit 46 to the fuel processor 44. Conduit 56 extends from thesteam separator to the coolant pump 50 to enable the coolant pump todraw coolant from the steam separator. A blow down conduit 58 extendsfrom conduit 56 at a point downstream of the steam separator andupstream of the coolant pump. The blow down conduit has a blow downcontrol 60 for removing a portion of the aqueous coolant to controlsolids content by exhausting a portion of the coolant to exhaustconduits 40, 42.

The coolant pump 50 delivers pressurized coolant to manifold 62 viaconduit 64. A plurality of conduits for the aqueous coolant, asrepresented by the conduits 66, extend from the manifold 62 through thefuel cell stack. These conduits are regularly spaced along the length ofthe fuel cell stack, as shown for example in U.S. Pat. No. 4,233,369issued to Breault et al. entitled "Fuel Cell Cooler Assembly and EdgeSeal Means Therefor". Each conduit has an orifice 68 for controlling thedistribution of coolant from the manifold to the stack. The coolantwhich is passed through the stack via conduit 66 is collected bymanifold 70. Conduit 72 extends from manifold 70 to the heat exchanger52 to duct the aqueous coolant to the heat exchanger where heat isremoved from the coolant. Conduit 74 completes the cooling loop byextending from the heat exchanger to the steam separator 54.

The coolant recovery and supply system 18 includes a condenser 76, adegasifier 78, a boost pump 80, a water treatment system 82, and afeedwater pump 84.

The condenser receives cathode exhaust directly from the fuel cell stack12 via conduit 42 and combusted anode exhaust through the fuel processor14 via conduit 40. These conduits join together with blow down conduit58 downstream of the fuel processor 44 and duct the steam, cathodeexhaust and anode exhaust to the condenser.

The condenser removes heat from the steam, the cathode exhaust and theanode exhaust. As a result, steam from the cooling system and watervapor in the anode and cathode exhaust are condensed as water. Thecondensed water is flowed from the condenser via conduit 86 to thedegasifier 78.

The degasifier 78 is adapted by vent 88 to receive the condensed water.The degasifier has a source of pressurized air, as represented by airpump 90, for passing air through the vent to degasify the incomingwater.

The degasified water is collected below the vent at a location which isin flow communication with the boost pump 80. The boost pump 80 drawswater from the degasifier via conduit 92 and supplies high pressurewater through conduit 94 to the water treatment system 82. The watertreatment system removes suspended and dissolved contaminants from thewater by filtration and ion exchange demineralization respectively.Oxygen reduction, if desired, may be achieved by thermal steamdeaeration while organic removal is achieved by absorption filtrationsuch as is commonly performed with activated charcoal. The resultantwater effluent from the water treatment system is purified water.Conduit 96 extends from the water treatment system to supply purifiedwater to the feedwater pump 84.

A recirculation conduit 98 extends to the degasifier from conduit 96 forreturning a portion of the flow supplied by the boost pump to thedegasifier. The recirculation conduit divides the conduit 96 into afirst portion 96a in flow communication with the water treatment systemand a second portion 96b in flow communication with the feedwater pump84. The feedwater pump supplies aqueous coolant as needed to the coolingsystem to supply coolant as required. Conduit 100 extends from thefeedwater pump through check valve 102 to the cooling system for thispurpose. Conduit 100 might alternatively be in flow communication with ameans for disposing type II ferric hydrous oxide in the aqueous coolantlocation 26.

As shown, conduit 22 is a means for disposing type II ferric hydrousoxide in the aqueous coolant and is part of the auxiliary laboratorysystem 24. The auxiliary laboratory system includes a water tank 104, awater-steam deaerator 106 for storing supply water (normal temperatureabout 212° F., 100° C.) a heat exchanger 108 (normal temperature 140°F., 60° C.), a boost pump 112, a water treatment system 114 (like watertreatment system 82), a feedwater pump 116 and a check valve in conduit22 (not shown). In this particular application, the water tank andwater-steam deaerator are formed as a single unit. The auxiliary systemis adapted at the following locations to receive type II ferric hydrousoxide upstream of the water treatment system at the water tank L₁ or atthe fill port L₂ ; and, downstream of the water treatment system, forexample, at the feedwater pump at L₃.

FIG. 3 is an enlarged side elevation view of the orifice 68. The orificehas a maximum diameter D₁, a minimum diameter D₂ and a contractingtransition region between the maximum and minimum diameters 118. Thestippled area shows the contour of the orifice having, for example, aminimum diameter D₃ which might result from the accretion of particlesby deposition during operation of the fuel cell powerplant. As will berealized, the deposition of a large amount of particles will increasethe flow resistance of the orifice to the passage of coolant andconcomitantly the flow resistance of the fuel cell stack to the coolant.

FIG. 4 is an alternate embodiment of the fuel cell powerplant shown inFIG. 1 having means 122 for disposing ferric hydrous oxide in thecoolant. The means 122 includes means 124 for forming ferric hydrousoxide within the powerplant. The means 124 includes a tank 126, a sourceof process water and a source of heat. In the embodiment shown, thesource of process water is the coolant recovery and supply system 18.The source of heat is the fuel cell stack 12. Means for transferringheat from the fuel cell stack to the tank, such as the cooling system16, is in flow communication with the heated coolant via conduit 128 andflow control 132. The flow control is responsive to the temperature ofthe aqueous solution in the tank and the blowdown requirements for thesystem effluent. A by-pass conduit 133 is provided to by-pass flow asrequired.

FIG. 5 is an enlarged, schematic representation of the means 124 forforming the hydrous ferric oxide within the power plant. The tank 126has a first chamber 134, a second chamber 136 and a third chamber 138.The third chamber receives purified water from the water treatmentsystem via conduit 96a'. A heat exchanger 142, such as a coil of conduit128, is disposed in the third chamber to transfer heat to the purifiedwater and to make steam. The second and third chambers 136, 138 might beinsulated against the loss of heat to the first chamber.

The second chamber 136 receives purified water via conduit 96a" andsteam via conduit 144. Alternatively, steam might be provided directlyto the second chamber 136 by flowing aqueous coolant from the coolantflow path such as through conduit 128 and conduit 146 shown in phantom.

The second chamber 136 is adapted by a nozzle 148 to mix the steam fromconduit 144 and the process water from conduit 96a" and inject themixture into the chamber. One suitable nozzle for this purpose is the1/4J Siphon Spray Nozzle available from Spraying Systems, Inc., NorthAve., Wharton, ILL. 60187. After injection, the water collects in thetank leaving a steam-water interface 152. The water level in chamber 136is controlled by continuing over flow through a conduit 153 whichextends back to the degasifier through conduit 98. A portion of conduit153 is broken away for clarity. A vent 154 is provided for venting gasesreleased during the mixing process.

The second chamber 136 has a source of iron disposed in the tank, suchas rods 156 formed of an iron alloy or an iron containing liner 158 onthe walls of the tank. Conduit 96a'" for withdrawing process waterextends from the second chamber through the third chamber of the tankand thence to feedwater pump 84. A heat exchanger 162 is disposed in thefirst chamber 134. The heat exchanger is in flow communication viaconduits 164a and 164b with a source of coolant, such as the purifiedwater of conduit 96 and a supplemental heat exchanger (not shown), forremoving heat from the process water in conduit 96a'".

During actual operation of the fuel cell powerplant represented in FIG.1, the deposition of iron based compounds (that is, iron based oxidesand salts including magnetite, hematite, iron phosphates and othercompounds resulting from iron corrosion) is retarded by disposing in theaqueous coolant type II ferric hydrous oxide (that is, Fe₂ O₃.XH₂ O orFeOOH of a character that retards the deposition of iron basedcompounds). Experimental work has confirmed this result. A portion ofthis work is shown in FIG. 6.

The upper curve of FIG. 6 (curve A) is a graphical representation of thevariation with time of the equivalent diameter of the fuel cell stackDea under operative conditions. The lower curve (curve B) beginning atabout 2,000 hours shows the presence or absence of ferric hydrous oxidein the feedwater to the aqueous coolant which is supplied via auxiliarysystem 24 through conduit 22.

In general, FIG. 6. shows that the presence of suspended type II ferrichydrous oxide in the feedwater, as evidenced by a yellow coloredfeedwater filter stain reported in curve B, precedes slightly andgenerally corresponds to periods of stable or increasing equivalentdiameter as shown by curve A. Examples are the time periods II, IV, andVI. Conversely, the absence of ferric hydrous oxide in the feedwater, asevidenced by the lack of a yellow feedwater filter stain, corresponds toperiods of decreasing equivalent diameter such as periods I and V. Theeffect of injecting tyep II ferric hydrous oxide is not instantaneousand precedes for a short period the effect on the equivalent diameter ofthe fuel cell stack.

More particularly, curve A of FIG. 6 shows the equivalent diameter Deaof the fuel cell stack normalized by dividing by the initial equivalentdiameter Dei. The actual equivalent diameter is Dea and is equal to Deiat the time the fuel cell stack begins operation. Curve B shows theintensity and color produced by filtering the feedwater through 0.45micron filter paper in accordance with the techniques described by theBabcock and Wilcox membrane filter comparison charts. These charts arepublished by the Babcock and Wilcox Company, Power Generation Division,(copyright 1964, 1970) New York, N.Y. entitled "Membrane FilterComparison Chart (Fe₂ O₃.XH₂ O)"; "Membrane Filter Comparison Chart (Fe₂O₃.XH₂ O-Fe₃ O₄) 1.5:1"; "Membrane Filter Comparison Chart (Fe₂ O₃.XH₂O-Fe₃ O₄) 2:1"; "Membrane Filter Comparison Chart (Fe₂ O₃.XH₂ O-Fe₃ O₄)1:1"; and, "Membrane Filter Comparison Chart (Fe₃ O₄)". The material inthese charts is incorporated herein by reference.

The comparison of the stains left by the feedwater with the membranefilter comparison chart enables an estimation of the presence of theferric hydrous oxide in the feedwater to the cooling system. Forexample, the feedwater filter stains show that ferric hydrous oxide waspresent in the feedwater in amounts of about 50 to 200 parts perbillion. Similarly, when ferric hydrous oxide was absent, black ironoxides were frequently present in an amount of about 100 to less than 10parts per billion.

During the first 2,000 hours of operation in period I prior to theaddition of type II ferric hydrous oxide, the pressure drop through thestack increased and the effective diameter decreased as iron basedcompounds were deposited on the walls of the cooling system.

Prior to the addition of the type II ferric hydrous oxide, the coolantfor the fuel cell stack was an aqueous coolant and the feedwater waspurified water from a water treatment system. The purified water wastreated condensate water fed via conduit 100 which was deionized anddeoxygenated to a level of 40 to 200 parts per billion by variousprocedures which are well-known. This purified water will have a pHwhich is greater than 5.5 (preferably in a range from 5.5-8.0), aconductivity which is less than one micromho per centimeter, analkalinity level of less than 0.2 ppm as calcium carbonate, a solidscontent which is less than one part per million (1 ppm) including lessthan 200 parts per billion of iron based compounds other than type IIferric hydrous oxide.

The period of time Ia shows the effect which accompanies shutdown andstart-up of the powerplant during the period when the feedwater was onlypurified water. Shutdown and start-ups occurred together respectively attimes S₁ and S₂. Simultaneously with the start-ups, feedwater wassupplied via conduit 22. Type II ferric hydrous oxide was disposed inthe feedwater by injecting continuously from time A₁ a solution of typeII ferric hydrous oxide and purified water into the feedwater system.This feedwater solution (or feedwater) was fed through conduit 22 at asteady average flow rate of one-tenth of a gallon per minute (0.1 gpm)into the flow path 20 for the aqueous coolant having a steady stateaverage flow rate of five gallons per minute (5 gpm).

At time A₁, the feedwater was flowed to the water tank at L₁ upstream ofthe water treatment system 114. It is believed that the water treatmentsystem removed the ferric hydrous oxide from the aqueous coolant. Thisevidenced (curve B) by the absence of the yellow feedwater filter stain.As a result of the absence of ferric hydrous oxide, the equivalentdiameter decreased except for the temporary effect of shutdown andstart-up.

Just before the beginning of time period II at time A₂, the supply ofwater containing type II ferric hydrous oxide was injected into thefeedwater downstream of the water treatment system at location L₃, and,the water treatment system was made inactive. The feedwater yellowfilter stain reappeared and the equivalent diameter began to recover.

During period II, beginning at A₃, the point of injection for the ferrichydrous oxide was moved from downstream of the water treatment system toupstream of the water treatment system at the fill port L₂, and, thewater treatment system was kept inactive.

The feedwater yellow stain continued and the equivalent diameterremained stable.

During period III beginning at A₄, ferric hydrous oxide was still addedat location L₂ upstream of the water treatment system, but the watertreatment system was made active for a short period of time. Soon afterthe water treatment system was made active, the ferric hydrous oxidedisappeared from the feedwater as evidenced by the absence of thefeedwater yellow filter stain.

At time A₅ at the beginning of period IV, the point of injection wasmoved further upstream of the water treatment system to the tank L₁, andthe water treatment system was made inactive. The feedwater yellowfilter stain reappeared for all of period IV.

At time A₆ at the beginning of period V, injection continued at thewater tank L₁ upstream of the water treatment system, but now the watertreatment system was made active. The ferric hydrous oxide againdisappeared from the feedwater as evidenced by the absence of the yellowfilter stain during period V. Shortly after A₆, a six day shutdown wasfollowed by a start-up S₃. The effective diameter continued to increasefor a short period after the water treatment system was made active(possibly because of shutdown and start-up), but the yellow feedwaterfilter stain never reappeared and the equivalent diameter decreased forthe remainder of period V. This included time after A₇ when the point ofinjection of the type II ferric hydrous oxide was moved downstream tothe fill port at L₂ (but still upstream of the active water treatmentsystem).

The downstream water treatment system was finally made inactive at timeA₈ at the beginning of the period VI. The yellow filter stain reappearedand the equivalent diameter began to recover.

At time A₉, the powerplant went through a shutdown, start-up cycle S₄.During shutdown, the cooling system was drained and refilled with watercontaining type II ferric hydrous oxide. After A₁₀, the downstream watertreatment system was kept inactive while water containing type II ferrichydrous oxide was continuously at the fill port L₂. The yellow feedwaterfilter stain continued and the equivalent diameter remained stable.

The downstream water treatment system was made active again at A₁₁ untilA₁₅ when the water treatment system was by-passed. It is thought thewater treatment system was ineffective during this period because it nolonger removed the ferric hydrous oxide as evidenced by the continuedpresence of the feedwater yellow filter stain.

Type II ferric hydrous oxide continued to be added upstream of the watertreatment system at A₁₅ and A₁₆ followed by replacement of watertreatment system components. The new water treatment system componentsremoved the ferric hydrous oxides as evidenced by the absence of thefeedwater yellow stain.

Thus, the deposition of iron based compounds is retarded, in some caseseliminated, and in other cases reversed by injecting type II ferrichydrous oxide into the feedwater and thence into the aqueous coolant.

Subsequent tests showed that feedwater from the coolant recovery andsupply system that was added at location 26 corrected the depositingcharacteristics of the iron-based compounds if it contained as little asone part type II ferric hydrous oxide water to three parts purifiedwater.

The phenomena by which the retardation of the deposition of these ironbased compounds occurs is not well understood. One working hypothesis isas follows. The iron based compounds are charged (experience suggestsnegatively charged) and surrounded by a cloud of oppositely charged ionsto form an electrical double layer. Particles of type II ferric hydrousoxide react with the normally depositing iron based compounds todestabilize them by shrinking or eliminating the double layer, thuschanging the level of charge on the compounds. As a result, the ironbased compounds are coagulated or agglomerated by the ferric hydrousoxide to an extent that interferes with their deposition on the walls ofthe water conduit.

It is not certain whether the particles of ferric hydrous oxide of acharacter which retard deposition occur as an inorganic polymeric ion oras a colloidal particle. One critical parameter is believed to be thesize of the particle in solution. The size of the particle in solutionis related to the characteristic particle dimension Pd of the particlewhich is found by filtering the feedwater. The dimension Pd is expressedin units of length and is measured along a line extending between thetwo most widely separated points on the particle. Those ferric hydrousoxide compounds having a dimension Pd at the time of measurement whichis greater than one micron are not believed effective, while thosehaving a dimension up to about one tenth of a micron are known to beeffectfive. In particular, it is known that a particle dimension of upto 100 angstroms is the preferred characteristic particle dimension Pd.

These ferric hydrous oxide compounds may be made by any one of severalprocesses. One way is to make the ferric hydrous oxide particles insuspension form in two steps. A concentrated solution of long shelf life(larger than one week) is made at room temperature by slowly hydrolyzingiron in a 0.1 to 0.6 molar solution having a pH of about 1 to 2 forperiods up to 3 months. Prior to use, the concentrated solution isdiluted with distilled water to the degree desired to form the actuallyused solution of higher pH. The effective life of the diluted solutionis believed to be another critical parameter. The effective life isshorter than the shelf life of the concentrated solution and is believedto not exceed one week.

EXAMPLE

Ferric hydrous oxide particles of a character that retards deposition ofiron based compounds in a conduit were prepared in two steps as follows:

1. 20.2 grams of ferric nitrate Fe(NO₃)₃.9H₂ O were dissolved in asufficient amount of distilled water at room temperature to produce a pHof approximately 1.5. The solution was kept at room temperature for tendays prior to use to form a slowly hydrolyzed ferric nitrate solution.It is believed this formed a suspension/solution of hydrous iron oxideFeOOH (Fe₂ O₃.H₂ O) particles. Transmission electron diffractionspectroscopy of the residue left after evaporation of the water from asample showed alpha-FeOOH (goethite) compounds were present in theresidue. The characteristic particle dimension Pd of these goethitecompounds ranged from 50 to 100 angstroms. The goethite crystals mayexist in the solution also or they may form only during evaporation ofthe water. It is believed that if refrigerated, this solution can bestored for a long period of time (at least two months).

2. The second step takes place prior to use. Prior to testing, the pH ofthe concentrated solution made by step 1 was raised to about 3.5 to 4.0by diluting the concentrated solution with distilled water. It isbelieved that the diluted solution at this pH is slightly unstable anddeteriorates after storage for five or six days. It is believed that thesize of the FeOOH particles increases with time and that after five tosix days the size of the particles has increased to a size which rendersthe particles ineffective for the purposes of this invention.

The dilute solution was tested in an apparatus as described above. Theeffectiveness of the fresh, dilute solution (less than one week old) wasdemonstrated by reversing in fifty hours at least one half the decreasein equivalent diameter of a fuel cell stack that occurred during onehundred fifty hours of operation. The equivalent diameter was then heldconstant for a further period of one hundred fifty hours. It is believedthat this suspension/solution is effective for use in purified water asset forth above having a pH which is greater that 5.5. Water at a pHbelow 5.5 is not believed deesirable for cooling systems for suchpowerplants because of the increase in corrosion of the powerplant.

Another method for preparing ferric hydrous oxide may be used inconjunction with the apparatus shown in FIG. 4 and FIG. 5. Duringoperation of this embodiment, heat is transferred from the aqueouscoolant in conduit 128 to purified water in the third chamber 138 toraise steam. The steam is flowed via conduit 144 to the spray nozzle148. Purified water is flowed via conduit 96a" to the spray nozzle fromthe water treatment system. The purified water has a conductivity ofless than one micromho or microsiemen per centimeter (and in theembodiment shown will likely be less than 0.5 micromhos or microsiemensper centimeter), has a pH that is nearly neutral (5.5-8), has somedissolved carbon dioxide and has an oxygen concentration ofapproximately 7 parts per million. In addition, the water contains ironbased compounds typically in an amount of about 80-100 parts perbillion.

Dissolved iron (predominantly Fe++) is also present in the purifiedwater supplied to the second chamber because of corrosion in the coolantrecovery and supply system 18 and is present in the water contained inthe bottom of the second chamber 136 because of corrosion of the ironrods 156 and the iron containing liner 158.

The steam and purified water are turbulently mixed as a result ofpassing through the spray nozzle 148 and are sprayed into the secondchamber 136 toward the surface of the water. The second chamber isoperated at atmospheric pressure and the water in the chamber is at atemperature which promotes the formation of type II ferric hydrousoxide. In the embodiment shown, the water is at a temperature of about212° F. It is believed the temperature of the water should be in a rangeof about 180° F. to about 250° F.

As the mixture is sprayed into the second chamber, portions of dissolvedoxygen and carbon dioxide are released above the steam-water interface152. These gases are vented via vent 154. Because the second chamber isvented, the bottom of the tank has a low oxygen level and a pH thatpromotes the type of iron corrosion that forms ferrous ions. These ionsare thought to migrate toward the steam-water interface.

Releasing portions of the dissolved carbon dioxide from the watersprayed into the second chamber and venting the carbon dioxide throughvent 154 has a second, important benefit. Releasing the carbon dioxidecauses the pH of the water in the spray and in the tank at the surfaceof the water to increase slightly. The increase in pH forces dissolvediron from the water entering with the spray and possibly forcesdissolved iron from the water at the steam-water interface toprecipitate in the form of type II ferric hydrous oxide. It is thoughtthis action is promoted by agitation of the iron contained in the waterthrough the turbulent injection of the steam and water spray. The oxygencontent of the incoming water is also controlled by venting to avoidhaving too much oxygen present. If too much oxygen is present, theoxidation rate and subsequent precipitation is too rapid and has theundesirable result of forming Fe₂ O₃ instead of forming type II ferrichydrous oxide which is a desirable result.

After forming the water containing the type II ferric hydrous oxide(FeOOH or Fe₂ O₃.H₂ O of a character that retards deposition), the wateris cooled to a temperature of about 140° F. in a low oxygen atmosphere(less than 60 parts per billion) and, even though the reaction formingtype II ferric hydrous oxide may continue at that temperature, is thenready for use as feedwater in the powerplant.

Water containing type II ferric hydrous oxide which is not used duringthe lifetime of the prepared solution returns via a recirculationconduit 98 to the degasifier tank. The water is pumped through the watertreatment system bed to remove aged and ineffective ferric hydrous oxideand other compounds from the water and to supply additional purifiedwater to the apparatus shown in FIG. 5.

The ferric hydrous oxide additive is supplied to the coolant loopthrough the feedwater pump as needed to replace at least a portion ofthe water removed from the cooling system and water used to make steamfor the fuel processor 44. Aqueous coolant removed via the blow downcontrol is flowed to the ferric hydrous oxide generator for heat andthence to the condenser where the water is recovered and sent to thedegasifier.

The method of determining the freshness and characteristic particledimension associated with the retardation in deposition includes thesteps of:

1. Setting up an apparatus which accurately simulates the system havinga problem with the deposition of iron based compounds.

2. Filling the cooling system with purified water.

3. Establishing the initial equivalent diameter of the system.

4. Operating the cooling system: until a ten percent decrease in theeffective diameter occurs; and, until the rate of decrease isestablished by measuring the decrease as a function of time.

5. Adding an amount of ferric hydrous oxide to the water flowed throughthe orifice, the residue of the ferric hydrous oxide having particles ofa known characteristic particle dimension Pd.

6. Determining whether the deposition of iron based compounds has beenblocked by monitoring the pressure drop and flow rate to determinewhether the equivalent diameter decreases at the same rate (depositionnot halted), decreases at a much reduced rate (blockage of deposition)stays the same (completely blocks deposition), or increases (reversal ofthe deposition process). The process is repeated with ferric hydrousoxide particles in varying amounts and varying freshness having acharacteristic particle dimension which is smaller and smaller in sizeuntil finally the desired effect is observed.

Having established the effectiveness of the ferric hydrous oxideparticles, the particles are added to the aqueous coolant, thusincreasing the life of the cooling system and increasing the timebetween costly shutdowns and overhauls of the cooling systems.

Although the invention has been shown and described with respect todetailed embodiments thereof, it should be understood by those skilledin the art that various changes in form and detail thereof may be madewithout departing from the spirit and the scope of the claimedinvention.

We claim:
 1. A fuel cell powerplant having a fuel cell stack, a flowpath for an aqueous solution, a conduit for the aqueous solution throughwhich the flow path extends, and a component for transferring heat tothe aqueous solution, the aqueous solution including water and ironbased compounds which deposit on the interior of the conduit, the waterbeing capable of reacting with its environment to form iron basedcompounds, wherein the improvement comprises:an aqueous solutionconsisting essentially of water having a pH of about 5.5 to 8.5, havingan electrical conductivity which is less than or equal to one micromhoper centimeter, having a solids content which is less than one part permillion (1 ppm), the solids content including an amount of iron basedcompounds other than ferric hydrous oxide and the water furtherincluding ferric hydrous oxide of a character and of an amount thatretards the deposition of said iron based compounds on the interior ofthe conduit; and, a supply conduit in flow communication with acomponent of the fuel cell powerplant for receiving water from saidcomponent; and, means for disposing additional ferric hydrous oxide inthe aqueous solution of a character that retards the deposition of saidiron based compounds on the interior of the conduit which includes meansfor forming the additional ferric hydrous oxide by the controlledcorrosion of iron or an iron based compound, the means being disposedwithin the powerplant and receiving water from the supply conduit inflow communication with a component of the fuel cell powerplant andbeing in communication with a portion of the conduit for the aqueoussolution.
 2. The powerplant as claimed in claim 1 wherein said supplyconduit is a first supply conduit and the powerplant further includes asecond conduit for flowing at least a portion of said aqueous solutionto the means for forming additional ferric hydrous oxide.
 3. Thepowerplant as claimed in claim 1 wherein said supply conduit is a firstsupply conduit and the powerplant further includes a second conduit forflowing at least a portion of said aqueous solution to the means forforming additional ferric hydrous oxide to provide water for corrodingthe iron or iron based compound.
 4. The powerplant as claimed in claim 1wherein said supply conduit is a first supply conduit and the powerplantfurther includes a second conduit for flowing at least a portion of saidaqueous solution to the means for forming additional ferric hydrousoxide to provide heat for raising steam.
 5. The powerplant as claimed inclaim 2 wherein said supply conduit is a first supply conduit and thepowerplant further includes a second conduit for flowing at least aportion of said aqueous solution to the means for forming additionalferric hydrous oxide to provide steam to the means for formingadditional ferric hydrous oxide.
 6. The powerplant as claimed in claim 1wherein said flow path for an aqueous solution extends through the fuelcell stack, wherein the powerplant further includes a condenser forcondensing water from said flow path for an aqueous solution, andwherein the supply conduit for receiving water from the component of thepowerplant is in flow communication with the condenser to receivecondensed water vapor that was contained in said flow path for anaqueous solution that extends through the fuel cell stack.
 7. Thepowerplant as claimed in claims 2, 3, 4, or 5 wherein the powerplant hasa fuel cell stack which generates heat under operative conditions;wherein the means for forming the additional ferric hydrous oxideemploys steam for promoting the controlled corrosion of iron or an ironbased compound; and, wherein the powerplant further includes means fortransferring heat from the fuel cell stack to the means for formingferric hydrous oxide such that water is heated to a temperature ofapproximately one hundred degrees Celsius (100° C.) to raise steam forpromoting the controlled corrosion of iron or an iron based compound. 8.The powerplant as claimed in claim 3 wherein the means for disposingferric hydrous oxide in the water includes means for establishing alevel of oxygen dissolved in the water such that the water containsoxygen in an amount of 20 to 400 parts per billion and includes a meansfor demineralizing the water such that the water is neutral and has analkalinity level of less than 0.2 ppm as calcium carbonate and a solidscontent which is less than one part per million (1 ppm).